Because the highest power of X is 2, this is a Quadratic Equation

When factorised, these look like 

       (x + a)(x + b)                 where a & b are constants

To factorise this, we need to find 2 numbers which add together to make the coefficient (number in front of) X and multiply to make the number at the end.

So we need two numbers which add together to make +1 and multiply to make -12       +4 & -3

a & b in the form above are replace by these to numbers, so the answer is

(x+4)(x-3)